This invention relates in general to patient-connected electrical equipment. More specifically, this invention relates to providing an isolated power source which will function in the presence of strong magnetic and radio frequency (RF) fields, for example, such as those associated with nuclear magnetic resonance (NMR) apparatus to monitor physiological function, such as heart rate, of a living human or animal subject undergoing examination.
NMR has been developed as an imaging modality utilized to obtain images of anatomical features of human patients, for example. Such images depicting nuclear spin distribution (typically, protons associated with water and tissue), spin-lattice relaxation time T.sub.1, and/or spin-spin relaxation time T.sub.2 are of medical diagnostic value in determining the state of health of the tissue examined. Imaging data for constructing NMR images can be collected using one of many available techniques, such as multiple-angle projection reconstruction and Fourier transform (FT). Typically, such techniques comprise scanning the patient with a pulse sequence made up of a plurality of sequentially implemented views. Each view may include one or more NMR experiments, each of which comprises at least a radio-frequency pulse and a magnetic field gradient pulse to encode spatial information into the NMR signal.
In some situations, it is desirable to physiologically monitor the subject during an NMR scan. This may be necessitated for medical reasons, in the case of an infirm patient, or to acquire signals in response to a change in a physiological characteristic of the subject and to use the signal to control some aspect of the scanning process, such as cardiac gating or respiratory compensation.
Typically, physiological signals are acquired in a well-known manner using electrodes attached to the body of the subject. The electrodes are connected by leads and additional cable, as necessary, to suitable electrical apparatus, for example, an electrocardiogram (ECG) amplifier which may be part of a patient monitor system. NMR imaging systems, however, create a difficult environment for patient monitoring systems to operate. For example, strong, steady-state magnetic fields, RF transmit pulses and audio frequency gradient field pulses compound the problems of monitoring low level physiological signals while maintaining a high degree of isolation and interference rejection between the patient and earth ground.
The conventional approach to isolated patient monitoring systems consists, typically, of providing an isolated small signal amplifier system (i.e., the ECG amplifier) for direct connection to the physiologic transducers (i.e., ECG leads). The output of this amplifier system is then transmitted to the grounded (non-isolated) instrumentation by means of, for example, a telemetry system which maintains the patient isolation. The isolated system, however, still requires a power source; either batteries or an isolated power supply. Batteries are not preferred since they require periodic replacement and occasionally deplete prematurely. Power supplies designed for conventional patient monitoring applications are typically dc-dc converters employing ferrite transformers with isolated windings. While these devices provide ample, regulated dc voltages and maintain high 60 Hz isolation, the ferrite transformer components saturate in high magnetic fields (e.g., 1.5 T) thereby rendering them unusuable in an NMR environment.
Applicants herein have discovered and claimed as their invention a means for applying power continuously to an isolated patient monitoring system, while maintaining a high degree of electrical isolation and interference rejection.